1. Field of the Invention
The invention relates to a method for reduction annealing of iron powder, produced by water atomizing an iron melt and to an arrangement for carrying out the method.
2. Description of the Prior Art
It is known to pulverize iron melts into small melt particles by means of gas jets or water jets which are directed under high pressure to a flow of the melt, so that a finely particulate iron powder is produced as a consequence of the resulting quick cooling of the melt particles. If the atomizing medium used is not oxygen-free (for example, water) and the atomization is not carried out in an inert atmosphere, an oxide skin is formed on the individual iron particles. The oxide skin constitutes an obstacle for the further processing of the iron powder, for example, in sintering metallurgy. In addition, there exists an obstacle to further processing for many applications because of hardening of the iron particles which takes place because of the extremely quick cooling, even though the carbon content is low.
For eliminating these obstacles, the oxidized iron powder obtained from the melt atomization is usually subjected to an annealing treatment in reducing atmosphere (for example, DE 37 22 956 C1).
Used for this purpose are continuous furnaces, such as conveyor-type furnaces (compare U.S. Pat. No. 4,448,746), walking-beam furnaces, or roller-hearth annealing furnaces. In these furnaces, the iron powder is placed in a loose layer, for example, on a bowl-shaped support, in the annealing furnace at temperatures of 900.degree.-1,200.degree. C. (in the heated furnace wall), usually above 950.degree. C. Most of the time, a furnace atmosphere enriched with hydrogen is used for the reduction. It is also known from DE 37 22 956 C1 to reduce the consumption of hydrogen by introducing hydrocarbons (for example, natural gas) into the furnace by utilizing the effect of vapor reforming of the hydrocarbons.
The dwell time of the iron powder in the furnace depends, on the one hand, on the initial oxygen content and the desired final oxygen content, i.e., the required reduction work and, on the other hand, on the limiting conditions for the reduction, i.e., particularly the height of the layer of the iron powder, the intensity of the gas exchange, and the reduction temperature. It is essential that the hydrogen required for the reduction can completely penetrate the powder layer and the water vapor formed during the reduction can emerge from the powder layer and the furnace atmosphere. Annealing times of 1-2 hours duration are to be considered conventional. After annealing, the iron powder only has a low residual oxygen content of, for example, less than 0.2% by weight, and a soft-annealed structure.
The known iron powder reduction has the disadvantage that the reduction process requires a large amount of energy and is expensive in view of the hydrogen consumption. The long drill periods negatively affect the furnace throughput. In addition, because the primary powder particles bake together, an "iron powder cake" is formed which must essentially be dissolved again by a subsequent grinding treatment.
It is also known to carry out a direct reduction of iron oxides in a revolving cylindrical furnace. A revolving cylindrical furnace is generally understood to be a furnace with a tubular treatment space which is fired directly and is continuously rotated during use. The charged material continuously travels through the revolving cylindrical furnace. The materials to be used in the direct reduction of iron oxides are ore pieces and iron ore pellets. A relatively small fine portion can also be processed therewith. However, particularly pulverous iron ores cannot be used.
Contrary to the revolving cylindrical furnace, a continuously operating furnace with a rotating cylindrical treatment chamber space which is indirectly heated is generally called a drum furnace.
From DE 34 39 717 A1 it is known to use such a drum furnace for producing pulverous tungsten or molybdenum by calcining ammonium paratungstate or ammonium molybdate and producing tungsten oxide or molybdenum oxide. These oxides are then reduced with hydrogen to the corresponding pulverous metals. A reduction of water-atomized iron powder in a drum furnace has not yet become known. Since iron powder (particularly water-atomized iron powder) has the tendency to agglomerate to a significant extent, an expert ab initio had to consider a drum furnace unsuitable for the reduction of iron powder. This is because it would have to expected that the formation of iron lumps (due to the spattered grain shape of the powder) impairs the furnace operation in an impermissible manner and prevents a sufficient and uniform powder reduction. In addition, it would have had to be expected that fine portions of the iron powder would have to be continuously removed, together with the necessary gas exchange, for renewing the furnace atmosphere, which would have reduced the yield of the process and would have negatively affected the economic use thereof.